The present invention relates generally to a driving method for a liquid crystal panel, and more particularly to a driving method for producing a substantially uniform tone pattern.
A conventional driving method, commonly referred to as the voltage averaging method, is generally employed for driving a matrix type liquid crystal panel. The voltage averaging method successively applies a selection voltage to each scanning electrode during each frame of a cycle (i.e. period). At the same time that a selection voltage is applied to a scanning electrode (i.e. a selected scanning electrode), a lighting voltage or a non-lighting voltage is applied to each of the signal electrodes. The selection of each scanning electrode and concurrent application of a lighting voltage or non-lighting voltage to each signal electrodes is repeated each frame. A display of lit and unlit dots (pixels) forming a desired pattern on the liquid crystal panel is produced. By providing two or more frames per period and a method for changing which of the dots is to be lit and unlit during each frame, a display with a desired tone (i.e., tone pattern/display) is produced. As used herein, a tone is considered to be a shade of grey that is neither white nor black.
Conventional driving methods also typically invert the polarity of the voltage applied across the liquid crystal panel from frame to frame to prevent application of a DC voltage to the liquid crystal panel.
In another conventional driving method, hereinafter referred to as a first driving method, the display dots associated with the same scanning electrode in forming the tone pattern during each frame are in the same lit or non-lit display state (i.e. the phases of the flickering cycles of each display dot associated with each signal electrode are equalized). The lighting or non-lighting of the display dots within the tone pattern is not based on the position of the signal electrodes. In yet another conventional driving method, hereinafter referred to as the second driving method, the adjacent display dots associated with the same scanning electrode are in different display states, that is, lit and non-lit. Each display dot in the second driving method is based on a position of a signal electrode. In particular, the display states of the display dots associated with a scanning electrode are varied based on the position of the corresponding signal electrodes.
Referring now to FIGS. 1(a) and 1(b), the display contents of a liquid crystal panel for a first frame and a second frame in accordance with the first driving method are illustrated, respectively. For exemplary purposes only, the liquid crystal panel includes a plurality of scanning electrodes Y1-Y10 and a plurality of signal electrodes X1 and X10. Each display dot is associated with the intersection (i.e. overlapping) of one of the plurality of scanning electrodes with one of the plurality of signal electrodes. A symbol .largecircle. represents a non-1 a mark symbol represents a lit display dot. Scanning electrodes Y1-Y10 are selected in ascending numerical order (i.e., Y1-Y10). FIGS. 1(a) and 1(b) illustrate a square using a half tone display positioned at the center of the liquid crystal panel.
The display dots corresponding to the odd numbered scanning electrodes (Y3, Y5, Y7) are not lit during the first frame (FIG. 1a) and lit during the second frame (FIG. 1(b)). The display dots corresponding to the even numbered scanning electrodes (Y4, Y6, Y8) are lit during the first frame and not lit during the second frame. Therefore flickering of the square display is not based on whether the signal electrodes are even numbered or odd numbered since all of the signal electrodes for a particular scanning electrode are of the same phase (i.e., lit or not lit). The first driving method therefore equalizes phases of flickering cycles of display dots generated by the scanning electrodes, for each scanning electrode, without regard to position of any signal electrode.
Referring next to FIGS. 4(a) and 4(b), the display contents of a liquid crystal panel for a first frame and a second frame in accordance with the second driving method is illustrated, respectively. Similar to FIGS. 1(a) and 1(b), the liquid crystal panel includes scanning electrodes Y1-Y10 and signal electrodes X1-X10.
In FIG. 4(a), the display dots produced by the odd numbered scanning electrodes (Y3, Y5, Y7) intersecting with the odd signal electrodes (X3, X5, X7) and the display dots produced by the even numbered scanning electrodes (Y4, Y6, Y8) intersecting with the signal electrodes (X4, X6, X8) are not lit in the first frame - and are lit in the second frame. Alternatively, FIG. 4(a) can be viewed as the display dots produced by the even numbered scanning electrodes (Y4, Y6,Y8) intersecting the odd numbered signal electrodes (X3, X5, X7) and the display dots produced by the odd numbered scanning electrodes (Y3, Y5, Y7) intersecting the even numbered signal electrodes (X4, X6, X8) being lit in the first frame and not lit in the second frame. Flickering of the display contents as shown in FIGS. 4(a) and 4(b) is based on the odd numbered and even numbered signal electrodes having different phases (i.e., out of phase with each other) per frame. In other words, the second driving method varies the phases of flickering cycles of the display dots generated by the scanning electrodes according to the position of the signal electrodes.
The unevenness of the display produced by the first driving method, which results in the aforementioned flicker, arises from crosstalk commonly referred to as zebra crosstalk (i.e., a zebra display pattern). Such unevenness is minimized by employing a driving method such as disclosed in Japanese Patent Application No. 63-159914. When the selected scanning electrode is successively changed, a nonuniformity in the display results. The nonuniformity is based on a parameter I, that is, the difference between the number of display dots currently lit on a selected scanning electrode (hereinafter referred to as the lighting dots) and the number of lighting dots currently lit in the scanning electrode to be selected next. That is, when the selected scanning electrode changes from the n-th scanning electrode to the n+1 th scanning electrode, and where the number of lighting dots on the n-th scanning electrode is Non and the number of lighting dots on the n+1 th scanning electrode is Mon, parameter I is equal to Non - Mon. When I is negative (-), a voltage with one or more spikes is generated on each scanning electrode. The spikes point in the side/direction of the lighting voltage. When parameter I is positive (+), a voltage with one or more spikes is generated in the non-lighting voltage direction/side based on the magnitude (i.e., absolute value) of parameter I. As can be readily appreciated, for relatively large values of parameter I which last for a relatively long period of time the level of the effective voltage applied to the display can vary significantly based on the voltage spikes. An appreciable increase in nonuniformity of the tone display results.
Referring once again to FIGS. 1(a) and 1(b), the first and second frames which are respectively represented by FIGS. 1(a) and 1(b), represents one cycle (period). The first driving method shown in FIGS. 1(a) and 1(b) requires that all of the display dots within the tone pattern associated with one of the scanning electrodes Y3-Y8 be lit or maintained not lit during each frame. A large value of parameter I results. For example, parameter I equals a value of 6 when the selected scanning electrode is changed from Y4-Y5. A relatively large nonuniform tone display is produced.
As shown in FIGS. 4(a) and 4(b), parameter I using the second driving method is minimized. For example, when the selecting scanning electrode is changed from Y4-Y5, parameter I has a value 0. Production of a nonuniform tone display is substantially avoided. More particularly, when a scanning electrode is selected the charge and discharge rates of the electric charges of the display dots associated with odd numbered signal electrodes is equalized by the charge and discharge rates of the electric charges of display dots associated with the even numbered signal electrodes.
The second driving method nevertheless produces a non-uniform tone display when used to drive a multicolor liquid crystal panel having a filter of three or more colors. Such a panel requires that the electrodes be disposed relatively close to each other. Consequently, an element providing a driving waveform to the panel must be connected to more than one signal electrode. For example, the element is connected alternately to opposite ends (i.e., upper and lower ends) of the signal electrodes to reduce the number of connections of different electrical elements to the signal electrodes. Assembly of a liquid crystal panel based on such connections to the signal electrodes contributes to a nonuniform tone display when using the second driving method.
More particularly, a driving waveform would be applied to the odd numbered signal electrodes (X1, X3, X5, X7, X9) from the top of the panel and to the even numbered signal electrodes (X2, X4, X6, X8, X10) from the bottom of the panel. Application of the driving waveforms to the upper and lower ends of the signal electrodes based on their position in the liquid crystal panel can result in higher charge and discharge rates of the display dots associated with the odd numbered signal electrodes as compared to the charge and discharge rates of the display dots associated with the even numbered signal electrodes. A difference in the charge and discharge rates between the display dots associated with the odd and even numbered signal electrodes results.
The lower charge and discharge rates of the display dots associated with even numbered signal electrodes is based on the significant level of attenuation to the driving waveform. This attenuation is based on the driving waveform being supplied to the lower end of each even numbered signal electrode as compared to the upper end of each odd numbered signal electrode. The magnitude of the driving waveform supplied to the even numbered signal electrodes is attenuated by their resistance and the impedance of other components in the panel. The driving waveform supplied to the odd numbered signal electrodes, however, is slightly, if at all, attenuated since the driving waveform need not travel along the length of the signal electrode before reaching the display dot. The substantial difference in the magnitude of the driving waveform based on the position of the signal electrode produces a relatively large difference in the charge and discharge rates of the display dots associated with the odd numbered and even numbered signal electrodes leading to a nonuniform display when using the second driving method.
In yet another method for driving a liquid crystal panel disclosed in Japanese Patent Application No. 63-159914 and commonly referred to as inversion stringing, a nonuniform tone display can occur. In this method, the polarity of the voltage applied across the liquid crystal panel is inverted when switching from one selected scanning electrode to the next selected scanning electrode. The shape of the waveform undergoing such inversion can change and is based on a parameter F. Parameter F is equal to the difference between the sum of the number of lighting dots on the current selected scanning electrode and the number of lighting dots on the next selected scanning electrode and the number of display dots on a scanning electrode (i.e., the number of signal electrodes).
When parameter F is negative (-), a rounding of the voltage waveform applied to the scanning electrode occurs based on the magnitude (i.e., absolute value) of the parameter F immediately after selection of a scanning electrode is made. When the parameter F is positive (+), one or more spikes in the voltage magnitude of the driving waveform applied to the signal electrodes occurs based on the magnitude (i.e., absolute value) of parameter F. The spikes rise in the direction (side) of the lighting voltage. The level of the effective voltage applied to the display dots due to the spikes in and rounding of the waveforms supplied to the scanning and signal electrodes can significantly vary (i.e., be greatly uneven) resulting in a nonuniform tone display.
The nonuniformity of tone display produced by the liquid crystal panel from these different driving methods leads to appreciable color degradation.
Accordingly, it is desirable to provide a driving method for a liquid crystal panel which produces a uniform tone display without appreciable color degradation. The driving method should correct for nonuniformity in the tone display produced by zebra crosstalk as well as nonuniformity produced at the time of polarity inversion of the driving waveforms.